1. Field of the Invention
The present invention relates to an image sensor units and methods for use in an image reading apparatus such as a facsimile machine, image scanner, copier machine, or other document processing apparatuses. More particularly, the invention relates to a method and image reading unit that prevents an electric charge, which is formed on a feeding document and a contact glass, from accumulating to an excess amount so as to avoid the creation of electric noise due to atmospheric discharge of the electric charge.
2. Discussion of the Background
A conventional image reading apparatus, such as a facsimile machine, has a contact-type image sensor unit with a contact glass on which a subject document (i.e., a feeding document) is brought into contact. The contact glass is located in a document feeding path of the image reading apparatus in such way such that a light source in the image sensor unit illuminates a surface of the feeding document located above the contact glass with respect to the light source. A converging lens is used to converge light reflected from the surface of the feeding document through the contact glass and onto a photoelectric cell. The photoelectric cell receives the reflected light and converts the same to an image data. The image data is then transferred to a main circuit board by way of a transfer device and a lead wire (e.g., a signal line).
According to the conventional contact-type image reading apparatus, the contact glass also functions as a document feeding path (guide), such that the feeding document is brought into contact with the contact glass and rubs against the contact glass when the feeding document is transferred toward and away from the contact glass. This rubbing action causes friction between the contact glass and the feeding document, and thus produces electric charge.
As a result, and as identified by the present inventors, electric charge formed by the frictional contact of the feeding paper and the contact glass may migrate to a frame of the image sensor unit and collect thereon. The accumulated electric charge will remain there until discharged therefrom. One mode of discharge is atmospheric discharge in which the excess electric charge will jump from the frame into the air or to an adjacent device. As recognized by the present inventors, such atmospheric discharge events produce electric noise that may corrupt the data integrity of the image data passed through the lead wire. The net effect of the corrupting the image data is that a resulting image formed on a printed sheet will itself be corrupted "image errors", which are deviations between the reproduced image with respect to the original image (on the feeding document). An example of an "image error" is a black line formed in the reproduced image, where the black line was not present in the original image.
The present inventors have further identified that with modern document processing devices, which operate at increased document reading speeds, electrical noise resulting from electrical discharge of accumulated charge has become even more pronounced than older, slower systems and will affect the quality of reproduced images unless appropriate steps are taken to prevent the atmospheric discharges.